1. Field of the Invention
The present invention relates to a flat display device, and more particularly, to a liquid crystal display (LCD) device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving efficiency of an LCD panel by optimizing the properties of liquid crystal according to a temperature.
2. Discussion of the Related Art
Recently, flat display devices such as a liquid crystal display (LCD) device, a filled emission display (FED) device, an electro-luminescence display (ELD) device and a plasma display panel (PDP) have been actively researched and developed. Among these flat display devices, the LCD device has attracted great attentions. The LCD device includes liquid crystal having both fluidity of liquid and optical property of crystal, such that the LCD device applies an electric field to the liquid crystal to change an optical anisotropy. Also, the LCD device has thin profile and low power consumption, and therefore, is widely used for vehicles and color televisions as well as lap top computers, PDAs and the like.
In general, the LCD device includes an upper substrate for a color filter and a lower substrate for a thin film transistor array. The upper and lower substrates face each other and sandwich a liquid crystal layer that has a dielectric anisotropy. Moreover, the thin film transistor array has a plurality of thin film transistors formed in hundreds of thousands of pixels and switched on/off by pixel-select address lines. Thus, a voltage is applied to a corresponding pixel, and is maintained to a next address by a capacitor.
Hereinafter, an LCD device in a related art will be described with reference to the accompanying drawings. FIG. 1 is a cross-sectional view illustrating the LCD device in the related art. As shown in FIG. 1, the LCD device is provided with an LCD panel 8, a backlight 9 that provides light to the LCD panel 8, a case (not shown) that accommodates the backlight 9 and supports the LCD panel 8 and the backlight 9, and a bezel part of a stainless steel material that surrounds an edge except an effect area of displaying an image. The bezel part is fixed to an outside of the case. Herein, the LCD panel 8 includes a liquid crystal layer 5 having a dielectric anisotropy, upper and lower substrates 1 and 2, and polarizing layers 6 disposed on outer surfaces of the upper and lower substrates 1 and 2 to selectively transmit light in a specific direction. In the LCD device, it is possible to control the amount of light generated in the backlight 9 according to the alignment state of the liquid crystal layer 5 and the position of polarizing axis of the polarizing layer 6, thereby realizing a gray scale.
Although not shown, the upper substrate 1 of the LCD device includes R(red)/G(green)/B(blue) color filters arranged in order to display colors, a black matrix layer for dividing R/G/B cells and shielding the light, and a common electrode for applying the voltage to liquid crystal cells. On the other hand, the lower substrate 2 of the LCD device further includes a plurality of gate and data lines crossing each other to define pixel regions, a plurality of thin film transistors T formed at crossing points of the gate and data lines to control turning-on/off of voltage, and a pixel electrode connected with the thin film transistor T to apply the voltage to the liquid crystal layer.
In case of an IPS (in-plane switching) mode LCD device, the common electrode is formed to be parallel with the pixel electrode on the lower substrate 2, thereby generating an IPS mode electric field that is parallel with the two substrates 1 and 2. Moreover, alignment layers may be formed on inner surfaces of the upper and lower substrates 1 and 2 having the aforementioned various patterns, thereby initially aligning liquid crystal molecules of the liquid crystal layer 5 at a desired direction. Meanwhile, the liquid crystal layer 5 may be formed of a high molecular material, in which the alignment of the liquid crystal molecules of the liquid crystal layer 5 can be easily changed by an external electromagnetic field, heat, and external stress such as adsorption of matters. That is, the optical properties of the liquid crystal layer 5 vary with the application of the voltage.
The LCD panel does not emit light by itself, and thus requires an additional light source for emitting the light. Especially, in case of a transmitting type LCD device, it is necessary to provide the backlight on a rear surface of the LCD panel. However, recently, a direct type LCD device with a big size LCD panel has become popular, and this brings about the increase of the number of backlights. As a result, a surface temperature of the LCD panel increases undesirably.
FIG. 2 is a graph illustrating Δn of liquid crystal according to a temperature of the LCD device. FIG. 3 is a graph illustrating Δ∈ of liquid crystal according to a temperature of the LCD device. When the temperature of the LCD panel increases, a refractive anisotropy (Δn) value and a dielectric anisotropy (Δ∈) value of the liquid crystal decrease together, thereby reducing a luminance and a contrast ratio (C/R) of the LCD panel. Further, since a threshold voltage (Vth) is in inverse proportion to the dielectric anisotropy, the threshold voltage is also changed. Herein, the threshold voltage (Vth) indicates a driving voltage of the LCD device when a transmittance is at 10%. That is, on the assumption that an initial temperature of the LCD panel is at 20° C., the LCD panel is driven so that the temperature of the LCD panel is increased to 60° C. due to the heat of the backlight. In the related art, the properties of liquid crystal are controlled in correspondence to the temperature of about 20° C. Accordingly, when the temperature of the LCD panel is increased to 60° C., the refractive anisotropy (Δn) value and the dielectric anisotropy (Δ∈) value decrease as shown in FIGS. 2 and 3.
For example, in a case where liquid crystal is applied to the IPS mode LCD device, the properties of liquid crystal are largely affected by the temperature. As shown in FIG. 2, the refractive anisotropy value is ‘0.0779’ at the temperature of about 20° C., but becomes ‘0.0581’ at the temperature of about 60° C. On the other hand, as shown in FIG. 3, the dielectric anisotropy value is ‘7’ at the temperature of about 20° C., but becomes ‘4.1’ at the temperature of about 60° C. Thus, the temperature properties of the LCD panel greatly affect the refractive anisotropy and the dielectric anisotropy.
FIG. 4 is a graph illustrating how the luminance and the temperature of the LCD panel change with the elapse of time. As shown in FIG. 4, when the time passes away, the surface temperature of the LCD panel increases to about 45° C. whereas the luminance of the LCD panel decreases sharply because the properties of the liquid crystal are changed. In addition, when the luminance is decreased at a predetermined white voltage (Vmax), the contrast ratio also decreases.
FIG. 5 is a graph illustrating a relative transmittance according to temperatures of the LCD device, as well as simulation results of transmittance in due consideration of the properties of liquid crystal at the temperatures of 20° C. and 60° C. As shown in FIG. 5, when a retardation (Δnd) of the liquid crystal decreases, Vmax (voltage corresponding to Von) and Vth (voltage corresponding to Voff) are shifted in an increased direction. Also, the picture quality of the LCD panel deteriorates due to image sticking and cross talk generated by the change of transmittance according to the temperature. Due to the temperature increase of the LCD panel, the properties of liquid crystal are changed, thereby sharply decreasing the dielectric anisotropy and the refractive anisotropy. As a result, the picture quality deteriorates due to the decrease of luminance and contrast ratio C/R, the shift of the threshold voltage, and the image sticking.